Jotta kvasipartikkelien lukumäärää tunneliliitosten läheisyThe SI unit ampere will be tied to an agreed value of the elementary charge within few years. With this redefinition, the ampere can be realised by controlled transfer of individual electrons at a constant frequency. This provides a direct frequency-to-current conversion.

One promising candidate for a new current standard is the SINIS turnstile, which consists of a small normal metal island that is connected to two superconducting leads via insulating barriers. A major problem for these types of devices is the existence of nonequilibrium quasiparticles, which are generated during operation of the device. These quasiparticles may tunnel through the insulating barriers causing excess current. In order to reduce the number of quasiparticles in the vicinity of the junctions, the geometry of the leads must be optimised. The quasiparticle density can be significantly reduced by designing the leads in such a way that the quasiparticles may effectively diffuse away from the tunnel junction. In this work, we have studied a simple diffusion model for quasiparticle transport and fabricated SINIS turnstiles with varying lead thicknesses.

In this work, we have fabricated devices where the normal metal island is either Cu or AlMn. The Mn suppresses the superconducting properties of Al, resulting in a normal metal island that can be oxidised in order to fabricate tunnel junctions for devices with very thick superconducting leads.

We were able to demonstrate that AlMn based devices can be indeed fabricated and that their charging energy can exceed the superconducting gap, which is essential in order to suppress several error mechanisms. However, we were not able to verify whether having thicker leads has a significant effect on the quasiparticle density.